Toshiba GRZ100 B Series, Инструкция по эксплуатации

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397
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6 F 2 S 0 8 4 6
CT Requirement
The requirement for minimum CT knee-point voltage for GRZ100 is assessed for the following
three cases separately:
a) Stability for faults beyond the zone 1 reach point:
V
k
> k
1
× I
f_z1_max
× (R
ct
+ R
2
)
b) Stability for close-up reverse faults:
V
k
> k
2
× I
f_rev_max
× (R
ct
+ R
2
)
c) Dependability of tripping for close-up forward faults:
V
k
> k
3
×I
f_max
× (R
ct
+ R
2
)
where,
V
k
: Knee point voltage.
I
f_z1_max
: Maximum fault current at the zone 1 reach point.
I
f_rev_max
: Maximum close-up reverse fault current.
I
f_max
: Maximum close-up forward fault current.
R
ct
: Resistance of CT.
R
2
: Burden including connecting leads.
k
1
, k
2
, k
3
: Transient dimensioning factor
(All values refer to the CT secondary side)
The minimum requirement for V
k
is determined for each of the three cases and the highest of the
three results is used to dimension the CT. k
1
, k
2
and k
3
are chosen depending on the primary
system time constant as follows:
Transient dimensioning factor, k
a) Stability for faults
beyond the zone 1
reach point (I
f_z1_max
)
b) Stability for
close-up reverse
faults (I
f_rev_max
)
c) Dependability of
tripping for close-up
forward faults (I
f_max
)
Primary system time
constant, Td (ms)
k
1
k
2
k
3
< 35
6 2 2
< 50 7 3 2
< 75
8 6 2
< 100
8 6 2
< 150
8 6 2
Notes:
1. Knee-point voltage, V
k
, is defined according to IEC 60044-1 as the minimum sinusoidal
e.m.f. (r.m.s.) at rated power frequency when applied to the secondary terminals of the
transformer, all other terminals being open circuited, which when increased by 10%, causes
the r.m.s. exciting current to increase by no more than 50%.
2. In cases where CTs are specified as P-class protective current transformers according to IEC
60044-1 (e.g. 5P10, 5P20 etc.), the knee point voltage can be approximated as follows:
V
k
≈
0.8 × n × I
n
× (R
ct
+ R
VA
)
where,